To release the "replacement" 810/R, you find the allen wrench that fits
the allen screw in the bottom of the face (it is recessed back into the
hole about 1/4 inch). Unscrew the allen screw and the unit should begin
to back out as you turn the wrench, You then turn the screw (about 10
turns) until the unit is free to pull out. If you strip this screw, you
are screwed. Be sure you have the right allen wrench before you bung it
up. It fits snugly into the screw.

Slight correction. I was thinking of the Narco transponder mounting
above. It has a threaded screw that turns into a threaded nut on the
back of the tray.

The replacement (810/R) is a bit different in that it releases by
unscrewing the allen screw only a couple of turns at which time it
should "unload" or become very east to turn. While holding slight
counterclockwise tension on the screw, you can pull the radio out.

There is a tab-like dog on the threaded screw that, when the screw is
turned clockwise the dog eventually twists into the small cutout in the
tray and hits a stop. Once it can no longer spin freely, it "rides" the
threaded portion of the screw and starts to draw the radio in tight.
When you unscrew it, it releases the radio and spins into the radio and
clears the tray. If you hold a little tension at that point, it won't
fall back down and catch the tray.

Impossible to visualize via this description I'm sure. One look and it
becomes quite clear.

Good Luck,
Mike